Copper(I) complexes of N-(aryl)pyridine-2-aldimines: Spectral, electrochemical and catalytic properties

Article abstract of DOI:10.1016/j.poly.2011.05.037

The reactions of N-(aryl)pyridine-2-aldimines (L-R; R = OCH₃, CH₃, H, Cl and NO₂), derived from pyridine-2-aldehyde and para-substituted anilines, with CuI in methanol under ambient conditions afford a series of brown comp

Full text of DOI:10.1016/j.poly.2011.05.037

Polyhedron 30 (2011) 2438–2443
Contents lists available at ScienceDirect
Polyhedron
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Copper(I) complexes of N-(aryl)pyridine-2-aldimines: Spectral, electrochemical
and catalytic properties
⇑
Dipravath Kumar Seth, Samaresh Bhattacharya
Department of Chemistry, Inorganic Chemistry Section, Jadavpur University, Kolkata 700 032, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The reactions of N-(aryl)pyridine-2-aldimines (L–R; R = OCH3, CH3, H, Cl and NO2), derived from pyridine-
2-aldehyde and para-substituted anilines, with CuI in methanol under ambient conditions afford a series
of brown complexes of the type [{Cu(L–R)I}2]. The structure of the [{Cu(L–OCH3)I}2] complex has been
determined by X-ray crystallography. In these dimeric complexes the two copper centers are linked
through an iodo-bridge, and the L–R ligands are coordinated to the metal center through the pyridine-
nitrogen and imine-nitrogen. All the complexes show characteristic 1H NMR signals and intense MLCT
transitions in the visible region. These complexes also show an emission near 465 nm, whilst they are
Received 4 May 2011
Accepted 26 May 2011
Available online 12 July 2011
Keywords:
N-(aryl)pyridine-2-aldimines
Copper(I) complexes
Crystal structure
Spectral properties
Catalytic activity
excited at 340 nm, with relatively poor quantum yields (u ꢀ0.002 at 298 K). Cyclic voltammetry on all
the complexes shows two successive Cu(I)–Cu(II) oxidations on the positive side of SCE, and a reduction
of the coordinated imine ligand on the negative side. These copper(I) complexes are found to efficiently
catalyze Suzuki type C–C coupling reactions.
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
2-aldimines with cuprous iodide indeed afforded a family of stable
complexes of copper(I), and herein we describe the chemistry of
The coordination chemistry of copper(I) has been attracting
considerable current attention [1–5], particularly because of the
fascinating fluorescence properties exhibited by copper(I) com-
plexes [6–8], as well as the growing interest in their use as cata-
lysts in several coupling reactions [9]. The coordination
environment around the copper(I) center plays a key role in tuning
the properties of the complexes, and hence complexation of cop-
per(I) by judiciously chosen ligands is of significant importance.
The primary objective of the present study was to synthesize stable
complexes of copper(I) and explore their luminescence and
catalytic properties. In order to stabilize copper in its +1 oxidation
state, a group of Schiff base ligands, viz. N-(aryl)pyridine-2-aldi-
mines derived from pyridine-2-aldehyde and para-substituted
anilines, were selected. These ligands are abbreviated in general
as L–R, where R stands for the para-substituent (R = OCH3, CH3,
H, Cl and NO2). The selected ligands have two soft donor sites,
viz. the pyridine-nitrogen and the imine-nitrogen, and hence were
expected to stabilize the +1 state of copper. These ligands are
known to bind to soft metal centers as a bidentate N,N-donor form-
ing stable five-membered chelate ring (A) [10]. Cuprous iodide was
chosen as the source of copper(I), particularly because coordina-
tion by the soft iodo ligand was also expected to stabilize the
copper(I) center. The reaction of the selected N-(aryl)pyridine-
these copper(I) complexes, with special reference to their spectral,
electrochemical and catalytic properties.
N
N
N
N
M
R= OCH3, CH3,
H, Cl, NO2
R
R
A
L-R
2. Experimental
2.1. Materials
Commercial CuI was obtained from Loba Chemie, India. Aniline
and the four para-substituted anilines were purchased from Merck,
India. Pyridine-2-aldehyde was obtained from Alfa Aesar, UK. The
N-(aryl)pyridine-2-aldimines (L–R, R = OCH3, CH3, H, Cl and NO2)
were prepared by the reaction of pyridine-2-aldehyde with the
respective aniline in ethanol. Tetrabutylammonium hexafluoro-
phosphate (TBHP) procured from Aldrich, and AR grade acetonitrile
procured from Merck (India) were used in electrochemical work.
⇑
Corresponding author. Tel./fax: +91 33 24146223.
E-mail address: samaresh_b@hotmail.com (S. Bhattacharya).
0277-5387/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2011.05.037

D.K. Seth, S. Bhattacharya / Polyhedron 30 (2011) 2438–2443
2439
Table 1
All other chemicals and solvents were reagent grade commercial
materials and were used as received.
Crystallographic data for [{Cu(L–OCH3)I}2].
Empirical formula
Formula weight
C26H24N4O2I2Cu2
805.39
2.2. Preparation of the complexes
Space group
a (Å)
b (Å)
Orthorhombic, Pbca
14.245(4)
9.949(3)
19.670(6)
2787.7(14)
4
All the [{Cu(L–R)I}2] complexes were synthesized by following a
general procedure. Specific details are given below for a particular
complex.
c (Å)
V (Å3)
Z
k (Å)
0.71073
2.2.1. [{Cu(L–OCH3)I}2]
Crystal size (mm3)
0.14 ꢂ 0.21 ꢂ 0.24
To a solution of L–OCH3 (225 mg, 1.05 mmol) in methanol
(30 mL) was added CuI (200 mg, 1.05 mmol), and the mixture
was stirred for 5 h to yield a brown precipitate, which was col-
lected by filtration, washed with cold methanol and dried in air.
Yield: 90%. Anal. Calc. for C13H12O1N2ICu: C, 38.70; H, 2.97; N,
6.94. Found: C, 38.75; H, 2.87; N, 6.90%. Mass: 678, [MꢁI]+. 1H
NMR in d6-DMSO, d ppm1: 3.74 (3H), 7.05 (2H), 7.81 (1H), 7.87
(2H), 8.05 (1H), 8.22 (1H); 8.77 (1H), 9.16 (1H). IR, m cmꢁ1: 1590,
1502, 1462, 1439, 819, 738, 535.
T (K)
293
l
R1
(mmꢁ1
)
3.770
0.0385
0.0912
0.94
a
b
wR2
GOFc
P
P
a
b
c
R1
=
||Fo| ꢁ |Fc||/ |Fo|.
P
P
P
2
wR2 = [ {wðF2o ꢁ F2c Þ }/ {w(F2o)}]1/2
.
2
GOF = [ (wðF2o ꢁ Fc2Þ )/(M ꢁ N)]1/2, where M is the number
of reflections and N is the number of parameters refined.
2.2.2. [{Cu(L–CH3)I}2]
H, C, N and O, and the SDD basis set for Cu and I. Electrochemical
measurements were made using a CH Instruments model 600A
electrochemical analyzer. A platinum disc working electrode, a
platinum wire auxiliary electrode and an aqueous saturated calo-
mel reference electrode (SCE) were used in the cyclic voltammetry
experiments. All electrochemical experiments were performed
under a dinitrogen atmosphere. All electrochemical data were
collected at 298 K and are uncorrected for junction potentials.
Yield: 94%. Anal. Calc. for C13H12N2ICu: C, 40.13; H, 3.10; N, 7.23.
Found: C, 40.15; H, 3.09; N, 7.33%. Mass: 646, [MꢁI]+. 1H NMR in
d6-DMSO, d ppm: 2.50 (3H); 7.07 (2H), 7.79 (1H), 7.88 (2H), 8.06
(1H), 8.20 (1H); 8.79 (1H), 9.18 (1H). IR,
1464, 1439, 818, 739, 536.
m
cmꢁ1: 1593, 1506,
2.2.3. [{Cu(L–H)I}2]
Yield: 91%. Anal. Calc. for C12H10N2ICu: C, 38.60; H, 2.68; N, 7.50.
Found: C, 38.68; H, 2.72; N, 7.48%. Mass: 618, [MꢁI]+. 1H NMR in
d6-DMSO, d ppm: 7.09 (2H), 7.78 (1H), 7.18 (1H), 7.89 (2H), 8.07
(1H), 8.20 (1H); 8.77 (1H), 9.19 (1H). IR,
1461, 1438, 817, 737, 536.
2.4. Crystallography
m
cmꢁ1: 1590, 1505,
Single crystals of [{Cu(L–OCH3)I}2] were obtained by slow evap-
oration of solvent from a solution of the complex in 1:1 acetoni-
trile–acetone. Selected crystal data and data collection
parameters are given in Table 1. Data were collected on a Bruker
SMART Apex CCD area detector using graphite monochromated
2.2.4. [{Cu(L–Cl)I}2]
Yield: 90%. Anal. Calc. for C12H9N2ClICu: C, 35.33; H, 2.20; N,
6.87. Found: C, 35.48; H, 2.23; N, 6.85%. Mass: 687, [MꢁI]+. 1H
NMR in d6-DMSO, d ppm: 7.10 (2H), 7.80 (1H), 7.89 (2H), 8.07
(1H), 8.21 (1H); 8.78 (1H), 9.22 (1H). IR,
1460, 1438, 819, 739, 537.
Mo Ka radiation (k = 0.71073 Å). X-ray data reduction, structure
solution and refinement were done using SHELXS-97 and SHELXL-97
programs [12]. The structure was solved by direct method.
m
cmꢁ1: 1592, 1506,
2.5. Catalysis: general procedure for the Suzuki coupling reactions
2.2.5. [{Cu(L–NO2)I}2]
Yield: 92%. Anal. Calc. for C12H9O2N3ICu: C, 34.44; H, 2.15; N,
10.04. Found: C, 34.48; H, 2.18; N, 10.10%. Mass: 708, [MꢁI]+. 1H
NMR in d6-DMSO, d ppm: 7.11 (2H), 7.82 (1H), 7.91 (2H), 8.07
(1H), 8.20 (1H); 8.79 (1H), 9.21 (1H). IR,
1463, 1436, 820, 738, 535.
In a typical run, an oven-dried 10 mL round-bottom flask was
charged with a known mole percent of catalyst, Cs2CO3 (1.7 mmol),
phenylboronic acid (1.2 mmol) and aryl halide (1 mmol) with poly-
ethyleneglycol (4 mL). The flask was placed in a preheated oil bath
at the required temp. After the specified time the flask was re-
moved from the oil bath and water (20 mL) was added, followed
by extraction with ether (4 ꢂ 10 mL). The combined organic layers
were washed with water (3 ꢂ 10 mL), dried over anhydrous
Na2SO4 and filtered. Solvent was removed under vacuum. The res-
idue was dissolved in CDCl3 and analyzed by 1H NMR. Percent con-
versions were determined against the remaining aryl halide
[13–16].
m
cmꢁ1: 1590, 1505,
2.3. Physical measurements
Microanalyses (C, H and N) were performed using an Heraeus
Carlo Erba 1108 elemental analyzer. 1H NMR spectra were re-
corded in d6-DMSO solutions on a Bruker Avance DPX 300 NMR
spectrometer using TMS as the internal standard. IR spectra were
obtained on a Shimadzu FTIR-8300 spectrometer with samples
prepared as KBr pellets. Electronic spectra were recorded on a JAS-
CO V-570 spectrophotometer. Emission spectra were recorded on a
Perkin Elmer LS55 luminescence spectrometer. Optimization of
ground-state structures and energy calculations for all the com-
plexes were carried out by the density functional theory (DFT)
method using the GAUSSIAN 03 package [11], where B3LYP was cho-
sen as the basis function and the 631g(d,p) basis set was taken for
3. Results and discussion
3.1. Synthesis and structure
As delineated in Section 1, five N-(aryl)pyridine-2-aldimines
(L–R) with different substituents (R = OCH3, CH3, H, Cl and NO2)
have been chosen to study the influence of these substituents, if
any, on the redox properties of the resulting complexes. All the five
ligands have been found to react readily with CuI under ambient
conditions to afford a series of brown complexes of the type
1
The expected multiplicity of the 1H NMR signals did not resolve in d6-DMSO
solution.

2440
D.K. Seth, S. Bhattacharya / Polyhedron 30 (2011) 2438–2443
In the dimeric complex each copper(I) center has a distorted tetra-
hedral N2I2 coordination environment, as reflected in the bond
parameters around each metal center. The two Cu–I distances are
observed to be significantly different (2.5543(11) and 2.6489
(11) Å), and the observed difference is attributable to the different
nature (directly linked and bridging, respectively) of the two iodo
ligands for a particular copper(I) center [17]. In the central Cu2I2
core of the [{Cu(L–OCH3)I}2] complex, the Cuꢃ ꢃ ꢃCu and Iꢃ ꢃ ꢃI dis-
tances are found to be 2.582 and 4.518 Å respectively, which are
comparable to the reported values for similar systems [18,19]. It
is relevant to mention here that the Cuꢃ ꢃ ꢃCu and Iꢃ ꢃ ꢃI distances in
iodo-bridged dicopper complexes are found to vary significantly,
depending on the nature of the terminal ligands [17–22], and con-
trolling these distances is of considerable current interest [23]. The
bond parameters within the Cu(L–OCH3) fragment are all found to
be normal [10]. As all the [{Cu(L–R)I}2] complexes have been syn-
thesized similarly, and they show similar properties (vide infra),
the other four [{Cu(L–R)I}2] complexes (R – OCH3) are assumed
to have similar structures as the [{Cu(L–OCH3)I}2] complex.
3.2. Spectral properties
All the [{Cu(L–R)I}2] complexes are found to be diamagnetic,
which corresponds to the monovalent state of copper (d10, S = 0).
The 1H NMR spectra of these complexes have been recorded in
d6-DMSO solution. A representative spectrum is shown in Fig. S1
(Supplementary material). Each complex shows the azomethine-
proton signal at around 8.8 ppm. Characteristic signals arising from
the pyridyl and pendent aryl fragments of the L–R ligand are also
observed in all the spectra. The infrared spectra of the [{Cu(L–
R)I}2] complexes show many bands of varying intensities within
the range 4000–400 cmꢁ1. Assignment of each individual band to
a specific vibration has not been attempted. However, several
sharp bands (e.g. near 1590, 1505, 1460, 1438, 817, 738 and
536 cmꢁ1) are observed in the [{Cu(L–R)I}2] complexes, which
were also found, at slightly different positions, in the uncoordi-
nated L–R ligands, and hence these are attributed to the coordi-
nated imine ligand. Of these bands, the one near 1590 cmꢁ1 is
assigned to the imine (C@N) stretch. In the uncoordinated ligand
the C@N stretch is observed at around 1620 cmꢁ1, and the ob-
Fig. 1. View of [{Cu(L–OCH3)I}2].
Table 2
Selected bond distances and bond angles for [{Cu(L–OCH3)I}2].
Bond distances (Å)
Cu(1)–I(1)
Cu(1)–I(1a)
Cu(1)–N(1)
Cu(1)–N(2)
2.5543(11)
2.6489(11)
2.101(4)
C(7)–N(1)
N(1)–C(1)
C(1)–C(2)
C(2)–N(2)
1.410(7)
1.284(7)
1.453(7)
1.345(7)
2.091(4)
Bond angles (°)
N(1)–Cu(1)–N2
I(1)–Cu(1)–I(1a)
I(1)–Cu(1)–N(1)
79.96(16)
120.53(3)
114.88(11)
I(1)–Cu(1)–N(2)
I(1a)–Cu(1)–N(1)
I(1a)–Cu(1)–N(2)
122.66(11)
111.18(11)
100.30(11)
[{Cu(L–R)I}2]. Preliminary characterization (microanalytical and
mass spectral) data are found to be in good agreement with the
composition of these complexes. In order to authenticate the
geometry around the metal center in these [{Cu(L–R)I}2] com-
plexes, as well as the binding mode of the L–R ligand, the structure
of a selected member of this family, viz. [{Cu(L–OCH3)I}2], has been
determined by X-ray crystallography. The structure is shown in
Fig. 1 and selected bond parameters are given in Table 2.
The structure shows that the L–OCH3 ligand is coordinated to
the metal center in the usual N,N-mode (A), and the iodide is also
coordinated to the metal center. Two such pyramidal Cu(L–OCH3)I
fragments, in both of which the copper is tricoordinated, are linked
together through the iodo-bridge in the [{Cu(L–OCH3)I}2] complex.
served shift to lower energy upon complexation is due to
p-back
bonding from the electron-rich copper(I) center to vacant imine
p⁄-orbital. The 1H NMR and infrared spectral data of the [{Cu(L–
R)I}2] complexes are therefore consistent with their composition.
The [{Cu(L–R)I}2] complexes are found to be insoluble in chloro-
form and dichloromethane, poorly soluble in methanol, ethanol
and acetone, but readily soluble in acetonitrile, dimethylformam-
ide and dimethylsulfoxide. Electronic spectra of the complexes
have been recorded in acetonitrile solution. Each complex shows
intense absorptions spanning the visible and ultraviolet regions.
The spectral data are presented in Table 3 and a selected spectrum
is displayed in Fig. 2. The absorptions in the ultraviolet region are
Table 3
Electronic absorption and emission spectral data.
Compound
Electronic spectral dataa
kmax, nm (
, Mꢁ1 cmꢁ1
Emission dataa
e
)
Excitation kmax (nm)
Emission kmax (nm)
Quantum yield (u)
[{Cu(L–OCH3)I}2]
[{Cu(L–CH3)I}2]
[{Cu(L–H)I}2]
[{Cu(L–Cl)I}2]
[{Cu(L–NO2)I}2]
340 (17 465), 291 (12 456), 285s (11 071), 244 (52 774), 207 (83 842)
336 (15 572), 291 (11 812), 281s (10 291), 243 (57 449), 207 (10 599)
332 (13 540), 288 (9586), 285s (9384), 244 (48 674), 210 (79 582)
326 (10 072), 292 (11 364), 283s (14 542), 243 (57 546), 207 (103 219)
332 (16 523), 299 (23 232), 288s (24 561), 244 (75 962), 207 (93 034)
340
340
340
340
340
465
466
463
465
470
2.34 ꢂ 10ꢁ3
2.24 ꢂ 10ꢁ3
2.12 ꢂ 10ꢁ3
2.00 ꢂ 10ꢁ3
2.27 ꢂ 10ꢁ3
a
s
In acetonitrile.
Shoulder.

D.K. Seth, S. Bhattacharya / Polyhedron 30 (2011) 2438–2443
2441
Fig. 2. Electronic absorption (_______) and emission (————) spectra of [{Cu(L–OCH3)I}2] in acetonitrile solution.
Table 4
Composition of selected molecular orbital of the complexes.
Complex
Contributing fragments % Contribution of fragments to
HOMO
LUMO
[{Cu(L–OCH3)I}2] Cu
I
73.67
21.01
04.96
73.98
20.88
04.91
73.82
20.92
04.93
73.75
21.09
04.88
74.03
21.18
04.63
20.22
02.87
76.45
21.07
02.71
75.97
20.35
02.63
76.36
19.98
03.01
76.70
20.58
02.94
75.81
L–OCH3
Cu
I
L–CH3
Cu
[{Cu(L–CH3)I}2]
[{Cu(L–H)I}2]
[{Cu(L–Cl)I}2]
[{Cu(L–NO2)I}2]
I
L–H
Cu
I
L–Cl
Cu
I
L–NO2
Table 5
Cyclic voltammetric data.a
Compound
E, V vs. SCE
[{Cu(L–OCH3)I}2]
[{Cu(L–CH3)I}2]
[{Cu(L–H)I}2]
[{Cu(L–Cl)I}2]
[{Cu(L–NO2)I}2]
0.35(153)b, 0.61(84)b, ꢁ0.63c
0.36(151)b, 0.62(84)b, ꢁ0.62c
0.37(150)b, 0.65(81)b, ꢁ0.59c
0.36(158)b, 0.64(81)b, ꢁ0.58c
0.33(154)b, 0.62(83)b, ꢁ0.56c
a
Solvent, acetonitrile; supporting electrolyte, TBHP; scan rate 50 mV sꢁ1
.
b
c
E1/2 value (
D
Ep value), where E1/2 = 0.5 (Epa + Epc) and
D
Ep = (Epa ꢁ Epc) in mV.
Epc value.
pied molecular orbital (LUMO) for the [{Cu(L–OCH3)I}2] complex is
shown in Fig. 3. The composition of these molecular orbitals for all
the complexes is given in Table 4. For all the complexes, the HOMO
has predominantly (>70%) metal character and a significant (>20%)
contribution comes from the coordinated iodide. The LUMO is
found to be delocalized mainly (>75%) over the imine ligand, with
ꢀ20% contribution from the metal center. Hence the lowest-energy
absorption may be assigned to a metal-to-ligand charge transfer
(MLCT) transition taking place from the filled d-orbital of copper
to the vacant p⁄-orbital of the imine ligand.
Fig. 3. Frontier orbital diagram of [{Cu(L–OCH3)I}2].
attributable to transitions within the ligand orbitals. To understand
the origin of the intense lowest-energy absorption, that is observed
within 326–340 nm, DFT calculations have been performed on the
[{Cu(L–R)I}2] complexes. The results are found to be qualitatively
similar for all the complexes. The electron distribution in the
highest occupied molecular orbital (HOMO) and lowest unoccu-

2442
D.K. Seth, S. Bhattacharya / Polyhedron 30 (2011) 2438–2443
Fig. 4. Cyclic voltammogram of [{Cu(L–CH3)I}2] in acetonitrile solution (0.1 M TBHP) at a scan rate of 50 mV sꢁ1
.
The high intensities (
e
, ꢀ104 Mꢁ1 cmꢁ1) of the MLCT transitions
coupling reactions [26,27], has prompted us to explore such cata-
lytic properties in the present series of copper(I) complexes. The
catalytic activity of the [{Cu(L–R)I}2] complexes has been exam-
ined for Suzuki type coupling of phenylboronic acid and aryl ha-
lides to yield the biphenyl product. Each of the five [{Cu(L–R)I}2]
complexes has been observed to catalyze the targeted coupling
reactions, and the catalytic efficiency of all the complexes are
found to be similar. Hence this discussion has been restricted only
to reactions using the [{Cu(L–OCH3)I}2] complex as a catalyst. Six
different aryl halides (viz. p-haloacetophenone (halo = chloro and
bromo), p-halobenzaldehyde and p-halobenzonitrile) have been
used for the Suzuki reaction with phenylboronic acid. The targeted
coupling reactions were achieved, under relatively mild experi-
mental condition, in reasonably high yield (Table 6). For example,
using 5 mol% of the [{Cu(L–OCH3)I}2] complex as a catalyst the Su-
zuki coupling between p-bromoacetophenone and phenylboronic
acid was achieved in excellent yield (entry 1). However, when p-
chloroacetophenone was used instead of p-bromoacetophenone,
the yield of the biphenyl product dropped to 62% (entry 4). Addi-
tionally, changing the para-substituent in the halobenzenes is also
have tempted us to explore the luminescence properties of these
copper(I) complexes and this has been carried out in acetonitrile
solution at ambient temperature (298 K). All the five [{Cu(L–
R)I}2] complexes have been found to display an emission in the vis-
ible region using an excitation wavelength of 340 nm. The emis-
sion spectral data are given in Table 3 and a selected spectrum is
shown in Fig. 2. Quantum yields (
evaluated with reference to [Ru(bpy)3]Cl2
[24,25]. The observed quantum yields (
u
) of these emissions have been
(u
= 0.028 at 298 K)
u
ꢀ0.002) indicate that
these [{Cu(L–R)I}2] complexes are relatively poor emitters at room
temperature [6–8].
3.3. Electrochemical properties
The redox activity of the complexes has been examined in ace-
tonitrile solution (0.1 M TBHP) by cyclic voltammetry. Each com-
plex shows two oxidative responses on the positive side of SCE
and a reductive response on the negative side. Voltammetric data
are presented in Table 5 and a selected voltammogram is shown
in Fig. 4. The two oxidative responses are assignable to two succes-
sive Cu(I)–Cu(II) oxidations. Support behind this assignment also
comes from the nature of the HOMO, which has predominantly
metal character. The separation between the two successive oxida-
tion-potentials is found to be around 200 mV, which indicates a
fairly good electronic communication between the two metal cen-
ters through the iodo-bridge. Both the oxidations are found to be
quasi-reversible in nature. The irreversible reductive response, ob-
served at around ꢁ0.6 V for all the five complexes, is attributable to
reduction of the imine-ligand as LUMO has predominant imine-li-
gand character. The voltammograms also show a sharp anodic
peak, associated with this reductive response, near 0.0 V, which
is due to generation of Cu(0) which gets absorbed on the electrode
surface, as evidenced from the narrow width of the anodic re-
sponse with a large peak current.
Table 6
Suzuki cross-coupling of aryl halides with phenylboronic acid.a
B(OH)2
X
[{Cu(L-OCH3)I}2] (5 mol%)
+
polyethyleneglycol, Cs2CO3,
120 ºC, 24 h
R
R
Entry
R
X
Conversionb (%)
1
2
3
4
5
6
COMe
CHO
CN
COMe
CHO
CN
Br
Br
Br
Cl
Cl
Cl
>99
90
87
62
51
44
3.4. Catalysis
a
Reaction conditions: aryl halide (1.0 mmol), phenylboronic acid (1.2 mmol),
The fact that copper(I) species have started receiving attention
as inexpensive catalysts for bringing about Suzuki type C–C cross
Cs2CO3 (1.7 mmol), Cu-catalyst, solvent (4 mL).
b
Determined by 1H NMR on the basis of the residual aryl halide [13–16].

D.K. Seth, S. Bhattacharya / Polyhedron 30 (2011) 2438–2443
2443
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Acknowledgments
The authors thank the reviewers for their constructive com-
ments, which have been helpful in preparing the revised manu-
script. Financial assistance received from the Department of
Science and Technology, New Delhi, India [Grant No. SR/S1/IC-29/
2009] is gratefully acknowledged. Dipravath Kumar Seth thanks
the Council of Scientific and Industrial Research, New Delhi, India,
for his fellowship [Grant No. 9/096(0511)/2006-EMR-I].
Appendix A. Supplementary data
CCDC 817828 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge via http://
www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.poly.2011.05.037.
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